Project Summary/Abstract Herpes simplex virus (HSV) keratitis is a leading cause of non-traumatic blindness in developed countries, with more than 400,000 cases in the USA, with approximately 50,000 new and recurring cases per year. HSV causes a variety of ocular diseases ranging from self-limiting dendritic epithelial keratitis, conjunctivitis, and blepharitis, to severe necrotizing stromal keratitis. In addition, HSV causes cold sores, genital sores, and is a leading cause of viral encephalitis. The HSV life cycle consists of a lytic phase at mucosal sites during which all virus genes are expressed, and a latent phase in neurons, during which gene expression is extremely limited. Latency is a permanent reservoir from which HSV periodically reactivates to cause severe mucocutaneous damage. Reactivation is very frequent and results in viral shedding. Latency renders HSV resistant to cure despite the availability of effective antiviral drugs, and no effective HSV vaccine exists. Better treatment and understanding of herpetic ocular disease is a stated goal of the NEI Corneal Diseases Program. Neurotropic herpesviruses rapidly shut off protein synthesis in infected cells. For HSV the major gene responsible for this shutoff is the virion host shutoff protein or vhs. Vhs is an RNAse, inducing rapid destabilization of host mRNAs. All neurotropic herpesviruses have a homolog of vhs although other classes of human herpesviruses and other important pathogens (e.g. SARS) destabilize host mRNA. This property is a critical virulence determinant. We showed that vhs activity plays an essential role in the infection and damage of the eye, in the development of periocular disease, and in the establishment of latency. Vhs activity alters the magnitude of the innate immune response and we identified functional domains within vhs, characterized a domain critical for its activity in the tegument, and shown that vhs is active in the nervous system. In addition viruses deficient in vhs are uniquely effective therapeutic vaccines (US Patent #5698431). Our current working hypothesis is that vhs determines the outcome of infection in vivo due to its ability to alter innate immune function and promote replication. Innate immunity is pivotal in determining the outcome of virus infection, and in the prevention of systemic and fatal infections in animal models. Most importantly, humans lacking interferon-mediated antiviral responses are highly susceptible to HSV. A better understanding of innate responses to virus infection is therefore needed. In addition, a better definition of vhs functions will define targets for therapeutic intervention against HSV diseases, as well as aid in design of HSV vaccines. Furthermore, there is emerging evidence that induction of host mRNA degradation is also a pathogenesis determinant for other human pathogens. Specific inhibitors of virus-induced RNA destabilization might therefore be of value as broad-spectrum antivirals. This work is therefore of broad interest and application.